The art of railway brakes includes two methods of retarding a railroad vehicle. One method is dynamic braking, in which the propulsion motors of a diesel-electric locomotive are used to generate electricity which is then dissipated through resistors. Another method is friction braking in which brake shoes are pressed against the treads of the wheels of the railroad vehicle, to provide a friction force which retards the wheels. Friction braking may also be provided by attaching a disc to a wheel or to an axle of the vehicle, and pressing brake shoes against the disc, thereby providing a friction force which retards the disc and hence retards the wheels. For friction braking in which brake shoes are applied to the wheels, heat is absorbed by the thermal mass of the wheels and then dissipated to the environment by conduction, convection and radiation from the wheels. For friction braking in which brake shoes are applied to discs, heat is absorbed by the thermal mass of the brake discs and then dissipated to the environment by conduction, convection and radiation from the discs.
In both cases, the amount of energy which can be absorbed is limited by the temperatures generated, since high temperatures may damage the brake shoes, or cause thermal stresses which cause warping or cracking of the wheels or brake discs. In some systems, the two methods are combined so that some of the heat is absorbed by the wheels and some is absorbed by the discs. By combining the two methods, more heat can be absorbed than can be absorbed by either method separately.
Such systems generally employ lever arrangements in which a single actuator, such as a brake cylinder, applies equal or proportional forces to a number of brake shoes, some of which may be applied to wheels, and some of which may be applied to discs which rotate with the wheels. Such systems may connect a single brake cylinder to brake shoes applied to all four wheels on a pair of axles. The lever arrangement which is typically used may apply equal force to the brake shoes on all four wheels.
Likewise, if each wheel has a disc associated with it, the lever arrangement may apply equal forces to brake shoes applied to all four discs. In some of these systems, brake shoes are applied to the rims of the brake discs. In others, brake shoes are applied to the faces of the discs. Application to the rim has an advantage over application to the face because the radius at which the friction force is generated is greater when it is applied to the rim than when it is applied to the face. Hence, the retarding torque exerted on the brake disc is greater when the shoe is applied with a given force to the rim than when the shoe is applied with the same force to the face.
For a railway vehicle, it is particularly important to apply the brake shoe at as great a radius as possible because the radius of the disc is limited by the required track clearance. In the United States this is 2.75 inches, so the radius of the brake disc must be at least 2.75 inches less than the radius of the wheel tread.
It is generally desirable for a brake system to provide a system for applying pressure to the brake shoe which provides mechanical advantage to amplify the total normal force between the brake shoe and the surface being retarded. For a brake shoe applied to the rim of a brake disc, prior attempts to accomplish this have been made by having the braking surface of the brake shoe have the form of a wedge, which is applied to a groove on the rim of the brake disc. With this configuration, the total normal force between the brake shoe and the brake disc is greater than the inward radial force applied to the shoe. An example of this is provided by U.S. Pat. No. 2,422,004.